1. Field of the Invention
This invention relates to lifting devices of the type in which a lifting pad is suspended from a beam. More particularly, the invention relates to apparatus for suspending a lifting pad from a generally horizontal beam.
2. Description of Related Art
In lifting flat articles such as flexible sheets and plates for movement from one location to another, it is common to use an arrangement in which one or more lifting pads are suspended from a horizontal beam, the horizontal beam in turn being suspended by a crane or similar arrangement. Where the articles are made of magnetic material, such pads may be electromagnets with a flat lower surface. Where the articles are non-magnetic or have a surface susceptible to damage from contact with an electromagnet, it is common to utilize a vacuum lifting pad, normally comprising a resilient ring which engages the surface and adheres thereto upon creation of a vacuum in the space surrounded by the ring.
Heretofore, lifting pads of the type described have been suspended from beams by lengths of chain or ball bolt arrangements, the latter comprising an elongated bolt having a ball at its lower end which is held in place by a socket formed in an upstanding boss attached to the top surface of the lifting pad; some ball bolt suspension arrangements are shown in the following listed U.S. Pat. Nos.: 3,152,828 (FIGS. 2 & 3); 3,183,032 (FIG. 2); 3,785,691 (FIG. 3); and 3,910,620 (FIG. 2).
Chain is sometimes satisfactory for suspending electromagnetic lifting pads, but having no compressive column strength it is generally unsatisfactory for vacuum lifting pads, which must often be pushed downward slightly on to the surface of the article being lifted so as to form a ring-to surface seal which will sustain the vacuum.
The ball bolt arrangement has sufficient column strength for vacuum lifting, but articulation or tilting of a lifting pad suspended by such an arrangement is typically limited to a maximum of about 7.degree. from the horizontal; when vacuum lifting a flexible or wavy sheet, especially a large sheet which requires two or more spaced lifting pads, loss of vacuum can occur if the sheet bends or has surface waviness to a degree which tilts the lifting pad past the limit imposed by the ball bolt arrangement.
I have developed suspension apparatus for a lifting pad which has sufficient column strength to be useful for vacuum lifting systems, allows the lifting pad to articulate or tilt up to 30.degree. or more from the horizontal, and includes secondary means for preventing unwanted distortion of the primary suspension component when the load on the lifting pad exceeds a predetermined level.